Hybrid vehicles, such as plug-in hybrid vehicles, may have two modes of operation: an engine-off mode and an engine-on mode. While in the engine-off mode, power to operate the vehicle may be supplied by stored electrical energy. While in the engine-on mode, the vehicle may operate using engine power. By switching between electrical and engine power sources, engine operation times may be reduced, thereby reducing overall carbon emissions from the vehicle. However, shorter engine operation times may lead to insufficient purging of fuel vapors from the vehicle's emission control system. Additionally, refueling and emission control system leak detection operations that are dependent on pressures and vacuums generated during engine operation may also be affected by the shorter engine operation times in hybrid vehicles.
In some conditions (e.g., city driving), an engine-off mode predominates and fuel may not be needed. Because fuel needs are reduced, fuel may remain in an onboard fuel tank for long time periods. As fuel remains in the fuel tank, it may be exposed to air within the tank and oxidize. Oxidation may occur when additional oxygen is ingested into the sealed environment of the fuel system. Oxidized fuel may be detrimental to plastics and metals found in a fuel system. In a closed system, such as a barrel of test fuel, the fuel does not age or deteriorate for a minimum of two years. Even after two years the fuel is still usable and combustion properties only begin to diminish.
For current plug-in hybrids or off-vehicle charge capable hybrid electric vehicle, attempts to protect against fuel oxidation and deterioration have involved burning fuel even when the vehicle is not demanding the gasoline powered internal combustion engine. A sealed or non-integrated refueling canister only system (NIRCOS) only allows air into the system typically in two methods, either mass may be removed from the fuel tank system, or a significant diurnal temperature or several thousand foot elevation change may occur. Removing mass may be accomplished by engine demand, such as in the example of utilizing an internal combustion engine even when power needs of the automobile are capable of being met by an engine-off mode.
Multiple embodiments of systems and methods for reducing fuel oxidation in a plug-in hybrid vehicle are provided. One method may include monitoring fuel tank pressure (FTP) and when below a threshold FTP, routing vapors to the fuel tank from a fuel system canister to maintain FTP at a desired pressure. Additionally, or alternatively, a portion or segment of a fuel tank may comprise a deformable material that may contract or expand with changes in FTP. Still further, a foam insert within the fuel tank may expand or contract to counteract changes in FTP. Also, vapor pressure within a fuel tank may be controlled by positive and negative pressure relief points that may employ expandable diaphragms. Still another approach may include a diaphragm chamber positioned between the tank and a fuel tank isolation valve (FTIV) pump to apply or remove pressure based on FTP. In yet another approach, a variable volume material may be used throughout a fuel tank that may expand or contract to counteract FTP, containing vapors at different barometric pressures, or temperatures.
In this way, it may be possible to mimic either an elevation change or significant temperature change to effectively remove vapor mass and reduce oxidation of onboard fuel. For example, such an approach may take advantage of NIRCOS or pressurized systems having pressure and vacuum relief for component protection. By expanding upon or subtly altering a pressure relief systems, it is possible to better manage fuel in the system, while reducing cost and packaging space.
Note that various systems and methods to control fuel tank pressure to reduce fuel oxidation in plug-in hybrid electric vehicles are disclosed. For example, in one example, a method comprises routing vapors from a fuel system canister to the fuel tank to maintain the fuel tank pressure at a desired pressure when fuel tank pressure is below a threshold. The routing of fuel vapors may be accomplished by a pump located between the fuel tank and the fuel system canister, or diverter valve from the canister allowing air into the canister to push vapor from the canister into the fuel tank. Again, such an approach enables fuel vapors to be managed in a way that reduces a need to run the engine only due to a need for fuel vapor purging, while also extending life of the fuel stored onboard by reducing the degree of pressure and temperature swings to which it is subjected.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.